1. Field of the Invention
This invention relates to an electromagnetically driven shutter in which a rotor is driven electromagnetically when a rotor winding arranged in a magnetic field is supplied with electric current and alternating movement of the rotor opens and closes the shutter blades.
2. Description of the Prior Art
In a known camera a single electromagnetic arrangement sequentially adjusts the focus by moving a photo-taking lens to a focus position in response to a sensor detecting an object distance and controls the exposure by varying the actuating time of the shutter blades on the basis of the object brightness and other exposure factors. Such a camera is, for example, disclosed in U.S. Pat. No. 4,355,872 and U.S. Pat. No. 4,243,309.
In such a conventional camera, a latch member stops the focus adjustment by engaging a focus adjusting control member. The focus adjusting and exposure controlling electromagnetic arrangement adjusts the focus with an electromagnet or solenoid that disengages the latch member from the control member with its electromagnetic force during the focus adjustment. Therefore, when the electromagnetic arrangement utilizes the rotor of an electromagnetically driven shutter of the type used in the prior art, a number of problems arise. Specifically, to assure a highly accurate and reliable focus adjustment it is necessary drastically to shorten the mechanical delay from the extinction of the electromagnetic force of the electromagnetic arrangement until the focus adjustment actually stops. This makes it necessary to bias the latch member with a relatively large resilient force. However, since it is far more difficult to increase the electromagnetic force of the rotor of an electromagnetically driven shutter than other electromagnetic devices, such as an electromagnet, solenoid, or the like, the increase in the strength of the resilient force of the latch member makes it difficult to prevent accidental actuation during the focus adjusting operation. Therefore, using the rotor of an electromagnetically driven shutter for the electromagnetic drive while leaving it unchanged from the device disclosed in the prior art may make it impossible to expect accurate control of the focus adjustment.
Of course, increasing the number of batteries loaded in the camera to increase the intensity of the current flowing into the rotor coil makes it possible to increase the electromagnetic force acting on the rotor to a level as high as that as other electromagnetic devices. However, this makes it impossible to limit the bulk and size of the camera to a minimum. Also, this is not a desirable means from the standpoint of conserving energy.
On the other hand, such an electromagnetically driven shutter can be made very thin along the axial direction of the photo-taking lens as compared with shutters using other electromagnetic drivers, such as electromagnets, solenoids or the like. Therefore, it is possible to construct a shutter of small overall size and to achieve an advance in the compactness of the camera. This results in a strong demand for development of an electromagnetically driven shutter capable of controlling the camera's focus adjusting operation.